This disclosure generally relates to systems and methods for calibrating the location of a workpiece or part relative to a coordinate system of a robot or other electro-mechanical machine that is guided by a computer program.
It is known to use robots (e.g., platform- and rail-mounted articulated arms, crawling vehicles, etc.) to automate composite non-destructive inspection (NDI) and assembly (e.g., drilling, trimming, bonding, fastening, painting, etc.) processes. These robotic methods have the potential to significantly reduce the times and costs of the various processes. Typically the robot comprises a base, a robotic arm, and interchangeable end effectors attachable to a distal end of the robotic arm. Depending on the automated function being performed, the end effector may comprise a non-destructive inspection sensor or sensor array or some other tool.
The production manufacturing of a large composite structure for an active airplane program needs to be done at a rate that meets schedule commitments. For example, it is known to fabricate fuselage sections made of composite material on an assembly line with high through-put. The finished fuselage sections need to undergo NDI at a high rate. In addition, high-speed NDI of stiffeners on the inside of many composite airplane fuselage or other stiffened sections is desirable in order to maintain the manufacturing rate.
The end effector of a computer-controlled machine must travel along a correct motion path when performing an operation on a workpiece. However, the correct motion path is a function of the location of the workpiece relative to the computer-controlled machine. For example, as successive workpieces are positioned and oriented at a NDI work station, the positions and orientations may vary from one workpiece to the next. To compensate for variations in position in orientation of the workpiece, the computer controlling the NDI sensor-carrying robot must be programmed with new motion paths for use in inspecting each new workpiece.
In a known NDI operation, robots holding ultrasonic arrays are in place to scan and inspect the Outer Mold Line (OML) and Inner Mold Line (IML) of the workpiece. In the case of a half-barrel fuselage section, the skin OML is inspected using one or more robots mounted outside, and stiffeners attached to the IML of the half-barrel fuselage section are inspected using one or more robots mounted inside. These robots can be on pedestals, permanent tracks, or have mobile bases.
Both the OML skin and the stiffeners on the IML must be inspected. The inspection robots must be oriented relative to the workpiece coordinate system for both. Robot motion control sequences are developed for the OML and IML inspections based on the location of the robot base relative to the half-barrel fuselage section, and it would be more cost effective if the motion sequences did not have to be re-programmed each time a new half-barrel fuselage section (of the same design) was moved into the workcell.
One challenge is that the location of the half-barrel fuselage section relative to the robot base is not accurately known, since in this configuration the location of the support fixturing relative to the robot base may vary from one half-barrel fuselage section to the next. Another challenge is that the three-dimensional (3-D) coordinates of the features of an unfinished half-barrel fuselage section (prior to installation of further supporting structure) may be either unavailable or variable as it is held in the support fixturing. These issues make it difficult to use existing measurement techniques to accurately determine the change in relative robot base position and the changes of specific feature locations between subsequent half-barrel fuselage sections.